Hopping Conduction in (Ni,Co,Mn)O₄ Prepared by Different Synthetic Routes: Conventional and Spark Plasma Sintering

(Ni,Co,Mn)O₄ (NMC) oxides were prepared by conventional sintering (CS) and spark plasma sintering (SPS) using micro and nanopowders. Small hopping polaron theory was used in order to investigate effect of processing routes on electrical properties of NMC oxides as negative temperature coefficient (NTC) thermistors. Also, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques were utilized to analyze compositional and structural effects on the electrical properties of NMC compounds. Hopping conduction in NMC prepared by SPS and CS using nanopowder occurs via variable range hopping (VRH) mechanism, however conduction in NMC prepared by CS using micropowder follows nearest neighboring hopping (NNH) mode. Hopping distance and activation energy for the VRH mode were calculated using corresponding physical model

Dissociation chemistry of hydrogen halides in water

To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X=F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and Moller-Plesset second-order perturbation methods (MP2). Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H2O)(n) and X-(H3O)(+)(H2O)(n-1) are calculated. The dissociated structures of HF and HCl can be formed for ngreater than or equal to4, while those of HBr and HI can be formed for ngreater than or equal to3. Among these, the dissociated structures of HX (X=Cl, Br, and I) are more stable than the undissociated ones for ngreater than or equal to4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable clusters including anharmonic frequencies are predicted to facilitate IR experimental studies. Undissociated systems have X-H stretching modes which are highly redshifted by hydration. Dissociated hydrogen halides show three characteristic OH stretching modes of hydronium moiety, which are redshifted from the OH stretching modes of water molecules. (C) 2004 American Institute of Physics.open11108108sciescopu

HF(H2O)(n) clusters with an excess electron: Ab initio study

The structures of electron-bound and neutral clusters of HF(H2O)n (n=1-3) were optimized at the level of second-order Moller-Plesset perturbation theory (MP2). Then, the energies were studied using the coupled cluster singles, doubles, and perturbative triples correction [CCSD(T)] method. The vertical detachment energies of the electron-bound clusters for n=1-3 are 60, 180, and similar to300 meV, respectively. In the case of the n=3, two structures are competing energetically. The electron-bound clusters for n=1 and 2 are 1.5 and 1.8 kcal/mol more stable than the neutral, while that for n=3 is 0.6-0.9 kcal/mol less stable. The excess electron is stabilized in the surface-bound state of the dipole oriented structures of the hydrated acid clusters. Vibrational spectra of the electron-bound clusters are discussed. (C) 2004 American Institute of Physics.open111616sciescopu

Size scaling of intramolecular charge transfer driven optical properties of substituted polyenes and polyynes

The origin of dramatic variation in optical polarizabilities of push-pull conjugated chains with respect to the conjugation length is discussed. Ab initio calculations of intramolecular charge transfer (ICT)-driven dipole moment (mu(CT)), polarizability (alpha(CT)), and first hyperpolarizability (beta(CT)) show that the values of alpha(CT)/mu(CT) and beta(CT)/mu(CT) change linearly and quadratically with respect to the conjugation length, respectively. The maximum ICT-driven coherence sizes of alpha(CT) and beta(CT) are consistent with the time-dependent densities to the first (rho((1))) and second (rho((2))) orders of the electric field obtained from the collective electronic oscillators method. (C) 2003 American Institute of Physics.open111919sciescopu

ZN(H2O)6(2+) IS VERY STABLE AMONG AQUA-ZN(II) IONS

Ab initio HF-SCF and MP2 calculations have been performed on Zn(H2O)n2+, where 4 less-than-or-equal-to n less-than-or-equal-to 8. The calculation with the Zn(8s4p2d)/O(4s2p1d)/H(2s) basis set shows that the fifth water molecule hydrating Zn (II) tends to have one more water molecule in the first hydration shell, while the seventh water molecule tends to be in the second hydration shell. Thus, we have provided evidence from ab initio calculations that hexaaqua-Zn (II) is very stable among aqua-Zn (II) ions. unless unusual thermodynamic conditions apply. The structure of the hexaaqua-Zn(II) has T(h) symmetry. The solvation energy contribution due to the primary hydration accounts for more than 50% of the experimental solvation energy of Zn (II). We also discuss the structure. hydration energy and spectra, comparing with experimental data.X1127sciescopu

Hydrogen detachment of the hexahydrated hydroiodic acid upon attaching an excess electron

A hydroiodic acid molecule is dissociated by more than four water molecules. Here, the effect of an excess electron on the hexahydated hydroiodic acid where the dissociated structure [H3O+(H2O)(5)I-] is much more stable than the undissociated one [(H2O)(6)HI], is investigated. Upon binding an excess electron (e(-)), the cluster releases a hydrogen radical and forms the stable hexahydrated iodide [(H2O)(6)I-] when the initial kinetic energy is above similar to 200 K, due to the small transition barrier (similar to 0.5 eV). The system with the hydrogen radical released, [(H2O)(6)I- + H center dot], is much more stable than the systems with an excess electron, [(H3O)(+){e(-)(H2O)5}I-] or [e(-)(H2O)(2)(H3O)(+)(H2O)(3)I-].X113sciescopu

WHAT IS THE GLOBAL MINIMUM ENERGY STRUCTURE OF THE WATER HEXAMER - THE IMPORTANCE OF NONADDITIVE INTERACTIONS

The global minimum energy structures of the water hexamer predicted by widely used analytic water potentials are very different from each other, while the cyclic hexamer does not appear to be a low-lying energy structure. However, high levels of ab initio calculation predict that a number of low-lying energy conformers including the cyclic conformer are almost all isoenergetic due to the balance of two-body and nonadditive interactions. For modeling of water potentials, we suggest that the binding energy of the dimer be between - 5.0 and -4.7 kcal (mol dimer)(-1), while the three-body corrections be taken into account to a large extent.open11108sciescopu